Just because someone (face or heel) carries the championship for a long time doesn't mean things will get boring, even if it's an underdog babyface like Bryan. As long as they book him in good rivalries and continue to maintain that atmosphere that Bryan could drop the title at any moment, the fans will always be on their toes and interested (just so long as he doesn't keep it for too long... like a year or so, but I doubt anyone honestly expects that to happen.) A three month or so reign wouldn't do justice imo because it'll probably take exactly that length of time just to completely wrap up his programs with HHH, Orton and Batista.... which would basically mean is that if he loses the title after only a few months, he never even gets to defend and retain it against a fresh opponent outside of the three guys whom he already beat the night he became champion. Meh.
I could see Bryan holding it to at least Summerslam, and even into the Fall for a bit after that before dropping it without the fans getting sick of it.